The friction material for use in brakes, such as disk brakes and drum brakes, or in clutches or the like is formed from starting materials such as a fibrous base material having a reinforcing function, a friction modifier which has a frictional function and serves to regulate the frictional performance of the fibrous base material, and a binder which integrates these ingredients together.
With the recent trend toward increases in performance and speed in vehicles, the role of brakes is becoming severer increasingly, and the brakes must have a sufficiently high friction coefficient (effectiveness). Furthermore, upon braking at high speeds, the friction material heats up to a high temperature and hence comes into a frictional state different from that in braking at low temperatures and low speeds. There is a desire for stable frictional properties which change little in friction coefficient with changing temperature.
It is presently known that to incorporate an appropriate amount of metallic fibers into a general friction material is effective in enhancing the strength of the friction material, stabilizing the friction coefficient thereof, maintaining the friction coefficient at high temperatures, improving the efficiency of heat dissipation, improving wear resistance, etc. A friction material produced while directing attention to metallic fibers for such use is disclosed in patent document 1, the friction material containing 5-10% by mass steel fibers, 5-10% by mass copper fibers having an average fiber length of 2-3 mm, and 2-5% by mass zinc powder having a particle diameter of 5-75 μm.
According to patent document 1, the friction material, due to the inclusion of copper fibers in an amount within the given range, can have an improved friction coefficient at low temperatures and can be inhibited from decreasing in friction coefficient at high temperatures during braking at high speeds. These effects are thought to be produced by the following mechanism. During friction between the friction material and the mating material (disk rotor), an adherent coating film is formed on the surface of the mating material because of the spreadability of a metal contained in the friction material and this coating film serves as a protective film. This coating film is thought to greatly contribute to low-temperature stabilization of friction coefficient and high-temperature retention of a high friction coefficient.
Meanwhile, there are cases where wear debris from the disk rotor or a metallic component contained in the friction material of the brake pad bites into the friction material and aggregate there into large metallic masses, which remain between the brake pad and the disk rotor. There are cases where the metallic masses thus formed by aggregation cause abnormal wear to the disk rotor (patent document 2).
At present, most of the metallic components contained in friction materials are metallic fibers mainly including steel fibers and copper fibers. There is a possibility that inclusion of such fibers in a large amount might cause the abnormal wear to the disk rotor.
In addition, since the copper component contained in friction materials is emitted as wear debris upon braking, influences thereof on the natural environment have been pointed out. Patent document 3 hence discloses a method for inhibiting the copper component in a friction material from dissolving away.